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Week 1 astronomy handout
Week 1 astronomy handout
Week 1 astronomy handout
Week 1 astronomy handout
Week 1 astronomy handout
Week 1 astronomy handout
Week 1 astronomy handout
Week 1 astronomy handout
Week 1 astronomy handout
Week 1 astronomy handout
Week 1 astronomy handout
Week 1 astronomy handout
Week 1 astronomy handout
Week 1 astronomy handout
Week 1 astronomy handout
Week 1 astronomy handout
Week 1 astronomy handout
Week 1 astronomy handout
Week 1 astronomy handout
Week 1 astronomy handout
Week 1 astronomy handout
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Week 1 astronomy handout

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  • 1. Huddersfield Astronomical Society Beginners Astronomy Course Week 1 "Put three grains of sand in a vast cathedral, and the cathedral will be more closely packed with sand than space is with stars" - Sir James Jeans If you were to travel through the vast emptiness of the universe you may eventually come across a collection of galaxies called Galaxy Group C7. One of the members of C7 is a large spiral galaxy containing over 100,000 million stars. Deep in the suburbs of this galaxy, about 30,000 light years from the centre lies a rather unspectacular yellow star. This is our sun. The only reason the sun lights up our sky is because it is so much closer than any other stars. When our part of the planet is facing away from the sun then the night comes and the sky is filled with thousands of distant suns. There is much to see in the night's sky. With the unaided eye, there are the constellations, shooting stars (or meteors), several of the planets (Mercury, Venus, Mars, Jupiter and Saturn are all visible to the unaided eye), and of course our moon. Also visible are some of the man-made interlopers in the night sky. Communication and weather satellites can often be seen (especially in the Summer) and you might see the International Space Station. Many people have access to a pair of binoculars, but never think to turn them on the night sky. A decent pair of binoculars is a great way of getting in to astronomy. Binoculars gather in more light than the human eye so not only do they bring things closer, you can actually see a lot of stars that are invisible to the unaided eye. Take a trip along the milky way with binoculars and you will see just how many stars surround us. Turn the binoculars on the moon and you can see many craters jump into focus. In the winter you can see the Orion Nebula (a stellar nursery) and star clusters such as the Pleiades and the Beehive cluster. In the summer months you can see the Great Cluster in Hercules. This is a tightly knit group of about one million stars that appear like a faint ball of light. Also visible with binoculars is the Great Galaxy in Andromeda. A huge galaxy about 2.5 million light years from earth. The Greatest Show on Earth Since the earliest civilisations, mankind has studied the stars. Heroes and gods have been placed among them. Great stories have been woven in the Page 1 of 21
  • 2. Huddersfield Astronomical Society constellations. Perseus, Andromeda, Hercules and Pegasus all look down on us from the sky. So many people throughout history have been enthralled by the stars, why not join them? Stars - Frequently asked questions What is a star ? Stars are essentially huge balls of gas (mainly hydrogen). The extaordinarily amounts of bright light and heat they emit is caused by nuclear reactions happening within the star. Perhaps the easiest way of describing what is happening inside a star is to explain how they are born: Stars are born in vast clouds of gas and dust. Dense clumps start to form in these clouds and these clumps start to exert gravitational pull. They start to suck in more gas from the surrounding cloud and this increases their gravitational pull which sucks in more gas. This cycle continues and just like a haystack in summer the inside of the protostar starts to get very hot. If enough material is is hoovered up from the surrounding gas and dust, the protostar will hit a critical point in its life. As the core temperature reaches about 10 million degrees centigrade it ignites, nuclear reactions begin and as they say in Hollywood a star is born. With its own gravity trying to collapse the star inwardly and the nuclear reactions trying to blow the star apart it manages to find an equilibrium and will settle down to burn for anything between a few million years and thousands of million years. (Our sun is about half way through its 10,000 million year life cycle.) Why do stars twinkle ? Stars twinkle because we are viewing them through thick layers of turbulent air in the Earth's atmosphere that slightly distort our view. The closer the star is to the horizon the more it appears to twinkle. This is because the light of stars near the horizon has to travel through more of the earth's atmosphere than the light of stars overhead and so is subject to more distortion. If you Page 2 of 21
  • 3. Huddersfield Astronomical Society were to view the stars from the moon the starlight would not have to travel through an atmosphere and the stars would appear rock steady. How many stars can be seen in the night sky ? On a clear night there are roughly 1500 to 2000 stars visible to the unaided eye, although this varies hugely depending on where you are. Cities and large towns cause light pollution that masks many of the fainter stars. If you want to see a really good starry night you need to get away from the cities and towns. The best view I've ever had of the night sky was on a river boat down the Amazon surrounded by millions of square miles of jungle. A cheaper option is to use a pair of binoculars to scan the night sky. You will then see many more stars than you ever could with the unaided eye. How many stars are there in the universe ? Okay it's number crunching time. The universe is made up of anywhere between 50 billion to 200 billion galaxies. Our galaxy contains about 100 billion stars so at a rough guess you are looking at: 100 billion multiplied by 100 billion. I can't possibly get my head around these numbers but a nicer way of putting it is: "There are more stars in the heavens than grains of sand on all the beaches on earth". Why is our sun so different from the other stars ? It isn't. The sun is a rather ordinary star. In fact astronomer's rather unkindly relegate it to the status of a yellow dwarf. The reason the sun dominates our sky is purely down to distance. At a mere 150 million km away the sun is sitting on our doorstep. After the sun the next nearest star is a red dwarf called Proxima Centauri which is over 250,000 times the distance away. If you were to travel out to Proxima Centauri and look back at the sun you would see it as a small faint yellow star. Is it true that stars are different colours ? Yes. Stars can be white, bluish-white, yellow, orange or red. It's colour depends on the temperature of the star. We normally associate red with hot and blue with cool but with stars it is the other way round. The hottest stars are white or bluish-white, then yellow, orange and the coolest stars are red. (cool being about 3,000 degrees in this case). Stars also differ greatly in size from giants that would swallow up half our solar system to superdense dwarf stars. When stars are in main sequence it is the larger stars that burn up their fuel more quickly and die. Smaller stars like our sun tend to conserve their fuel and last for much longer. What is a supernova ? Page 3 of 21
  • 4. Huddersfield Astronomical Society Stars that have managed to suck in enormous amounts of material at birth erupt into life as supergiants. These stars as well as being mammoths burn incredibly fiercely and after a relatively short lifetime will run out of fuel. As this happens the vast mass of the star collapses in on itself and the core becomes incredibly upset and volatile. This causes a tremendous reaction and the star blows itself apart. This explosion is called a supernova: the most cataclysmic event in the universe. Galaxies - Frequently asked questions What is a galaxy ? Stars are not spread out randomly across the universe. They tend to gather together in large groups held together by gravitational attraction. These large groups are galaxies. Every star that is visible when you look up at the night sky is part of our own galaxy. Galaxies themselves also tend to hang around in groups. Our own galaxy is part of a local group that consists of a few large systems and more than twenty smaller ones. Well beyond our local group astronomers know of galaxies that are 13,000 million light years away. How many stars are there in a galaxy ? Estimates for our own galaxy put the number of stars at about 100,000 million stars. However our own galaxy is one of the larger systems and most galaxies contain a lot less. What is the milky way ? This is the name given to the misty band of stars that can be seen on a good clear night. This misty band is caused by the shape of our galaxy. When we look at the milky way we are looking along the crowded plane of our galaxy. Although we can't make out individually the billions of stars we are looking at, their combined light makes a pearly glow across the night sky. Just to make matters a little bit confusing, the Milky Way is also the name we give to our own galaxy. Is our sun near the centre of our galaxy ? Galaxies are sometimes referred to as great cities of stars. Well if this is the case, our sun (and the earth) is in the leafy suburbs, about 25,000 light years from the bustling galactic centre. That's 25,000 x 9.46 million million km, in cosmic terms a short train ride away. Page 4 of 21
  • 5. Huddersfield Astronomical Society Meteors and Meteorites - Frequently asked questions What is a meteor ? A Meteor is the proper name for the streak of light that is usually called a shooting star. Meteors are caused when specks of dust about the size of grains of sand dash into the upper atmosphere from space. Moving at upto 45 kilometres per second the friction from travelling through air creates intense heat and they burn up giving off a bright light. Some of these bits of dust are singular pieces of debris flying through the solar system and getting swept up by the planets. Others travel in swarms of meteoric dust. Each time the earth passes through one of these swarms it causes a meteor shower. These groups of meteoric dust are left by passing comets which have had material blown off them by solar winds. The earth crosses the orbit of several comets every year and this is when we see meteor showers such as the Leonids or Perseids. How many meteors can be seen during a meteor shower ? The term meteor shower is a bit misleading. It suggests that meteors are falling like rain. This isn't the case. What you will find is an increase in the number of meteors over what you would see on an average night. Different showers have different intensities, ranging from 10 to over 100 meteors an hour. When is the best time to observe meteors ? Unfortunately for most of us who live near towns or cities a lot of the fainter meteors are washed out by light pollution. However during the Perseids shower (25th July to 18th August) or Leonids (15-19th November) you should still be able to see a meteor every few minutes. It is worth spending a bit of time outside during one of the these showers as a bright meteor is one of the most exciting sights in the night sky. What is the difference between a meteor and a meteorite ? Unlike a meteor which is a grain of dust, a meteorite is a fragment of rock that succeeds in reaching the ground. They are very uncommon and there are no reports of anyone being hurt by a meteorite. They are generally fairly small but one monster (the Hoba West meteorite) which fell in Namibia during prehistoric times weighs in at over 60 tonnes. Page 5 of 21
  • 6. Huddersfield Astronomical Society Comets - Frequently asked questions What is a comet ? Comets are often described as dirty snowballs. This isn't a bad description. The nucleus of a comet is made up of fragments of rock, ice and other frozen elements such as methane, ammonia and carbon dioxide. The nucleus is surrounded by a cloud of dust and gas. When a comet approaches the sun the head is illuminated by sunlight and some of the dust and gas is blown away by the solar winds. This creates the characteristic tail. How fast do comets move across the sky ? To be honest until I saw my first comet I thought they flashed across the sky and were gone in a few seconds. This is not the case. Comets appear static in the night's sky. Any apparent movement in position over the course of a night is mostly down to the earth's rotation. Over a period of nights the comet will be seen in a slightly different part of the sky as it continues on it's orbit around the sun. Does the tail always follow behind a comet ? Another misconception about comets is that as they hurtle through space the tail follows behind. In fact the tail is caused by the solar winds so it always points away from the sun. This means that as the comet moves away from the sun the tail is pushed out in front of it. Who or what are comets named after ? Comets are generally named after the person or persons that discovered them. The Hale-Bopp comet that appeared bright in our skies in 1997 was discovered in 1995 by Alan Hale and Thomas Bopp. Many comets are discovered by amateur astronomers. While the most powerful telescopes are focused on specific objects at very high power, it is the thousands of amateurs using smaller telescopes to scan the skies that usually see new comets for the first time. Astronomy is the last area of science where amateurs can make major scientific contributions from their own homes. Page 6 of 21
  • 7. Huddersfield Astronomical Society Is earth in danger of colliding with a comet ? The chances of earth being hit are very remote. Comets are very few in number in the inner solar system. All the orbits of known comets such as Halley's comet do not pose a danger to earth. It is possible that a new comet could be a threat but there is another defence system that protects our planet. The larger outer planets with their strong fields of gravity tend to hoover up a lot of wandering bodies such as comets and this is seen as one of the reasons that life has been able to develop on earth with relatively few interruptions from devastating impacts. One such occurence of a comet colliding with an outer planet happened in 1994 when the comet Shoemaker-Levy plunged into Jupiter. The bruise it gave the Jovian atmosphere was clearly visible from earth. The Moon - Frequently asked questions How was the moon created ? Up until the Apollo landings there were a number of competing theories about the origins of the moon. Study of moon rock samples brought back by the astronauts has led to the development of the impact theory. This suggests that the young earth collided with an object about the size of Mars. The debris thrown up from this collision went into orbit around the earth and formed into the moon. Why does the moon sometimes appear to be very large in the sky ? When you see the moon near the horizon it looks a lot bigger than when it is high up in the sky. This is due to the way the brain perceives size and distance. When the moon is close to the horizon the trees, houses etc indicate to the brain that the moon is a very long way away. When the moon is high in the sky there are no clues to the brain so it concludes that the moon is closer. Since an object's apparent distance determines its perceived size, the moon near the horizon looks twice the size compared to when it is high in the sky. What causes the different phases of the moon ? The moon is illuminated because it reflects the light from the sun. However only the half of the moon facing the sun is lit up. The side facing away from the sun is in darkness. How much of the moon we see from earth depends on the angle between the earth, moon and sun. As the moon orbits around the earth we see it grow from a thin crescent to a full disk (or full moon) and then shrink back to a thin crescent again before vanishing for a few days. Page 7 of 21
  • 8. Huddersfield Astronomical Society How does the moon cause tides on earth ? The gravitational pull of the moon causes the earth's oceans to bulge. This bulge is the high tide. In fact the moon causes the oceans to bulge in two places, the oceans facing the moon and the oceans facing away from the moon. The moon's gravity has a greater effect on the oceans facing the moon than on the earth itself. This causes the water directly under the moon to bulge. The moon pulls the rocky part of the earth more strongly than the water on the far side. This causes the oceans on the far side of the earth to get left behind and the water bulges there as well. Page 8 of 21
  • 9. Huddersfield Astronomical Society THE MOON - OUR CLOSEST NEIGHBOUR Most of the moons found in the solar system are very small in comparison to the planet they orbit. However the earth's moon is large in comparison (about one third the size of the earth) and many astronomers consider the earthmoon system to be a double planet. The moon orbits the earth every 27.5 days and its rotation time is the same so we always see the same side of the moon facing us. The first time mankind ever got a view of the dark side of the moon was when the Luna 3 space probe sent back photos in 1959. When looking at the moon with the unaided eye, two types of terrain can be seen. The lighter areas are the mountainous regions. These are known as the 'highlands'. The darker areas are the 'mare' regions. These are flat low lying regions. 'Mare' is the latin word for sea and at first it was thought that the darker areas of the surface were lunar oceans. They are actually large basins that filled up with lava which pushed its way to the surface 3 to 4 billion years ago. When looking at the moon through binoculars most of the craters appear to be Page 9 of 21
  • 10. Huddersfield Astronomical Society concentrated in the lighter areas. This is because early in its life the moon was bombarded with meteorites that were left over from the formation of the solar system. When lava flowed into the low lying regions it covered the craters in these basins. Since then the moon has been hit by relatively few large meteorites and the luna plains are not so cratered. Since the volcanism that ended about 3 billion years ago, not much has changed on the moon's surface. Apart from the infrequent small meteorite the recent changes have been man-made. There are the craters where probes and spent rocket stages have crashed into the surface and the footprints made by the astronauts themselves. In fact it is quite possible that these footprints could outlast any trace of mankind on earth. Moon Statistics Moon Equatorial radius (km) 1,737.4 Mean distance from Earth (km) 384,400 Rotational period (days) 27.32166 Orbital period (days) 27.32166 Average length of lunar day (days) 29.53059 Mean surface temperature (day) 107°C Mean surface temperature (night) -153°C Maximum surface temperature 123°C Minimum surface temperature -233°C Page 10 of 21
  • 11. Huddersfield Astronomical Society MERCURY, THE WINGED MESSENGER Mercury was known to the Romans as the messenger to the gods. It is the closest planet to the sun and hurtles round our star every 88 days. It is the second smallest planet, after Pluto. Mercury has an iron core that extends three quarters of the way to its surface. This suggests that at one time Mercury was considerably larger. It appears that a hefty blow from a stray object in its distant past may have blown much of its original rocky shell off into space. This is not the only scar Mercury displays from major collisions. The Caloris Basin is one of the largest impact craters in the solar system. At 800 miles across it is bigger than the British Isles. The impact was so great that the shock waves created rocky ridges on the other side of the planet. Although the Mercurian year is very short, the days (and nights) drag on. Mercury spins on its axis quite slowly and only fits in one and a half days to its year. If you were to visit the planet you would have to suffer interminably long nights where the temperature drops to -280 degrees Farenheit. At dawn the temperature would quickly soar and as the huge sun edged its way across the sky the barren landscape would heat up to 800 degrees farenheit. A visitor to the planet would also witness some of the most peculiar sunsets in the solar system. Sometimes Mercury's rapid motion round the sun outpaces it's genteel rotation. If this happens at sunset, the sun will dip below the horizon then pop back up again for a couple of days before sinking back down again plunging you into the long cold night. Mercury Statistics Mercury Equatorial radius (km) 2,439.7 Mean distance from the Sun (km) 57,910,000 Mean distance from the Sun (Earth = 1) 0.3871 Rotational period (days) 58.6462 Orbital period (days) 87.969 Mean surface temperature 179°C Maximum surface temperature 427°C Minimum surface temperature -173°C Atmospheric composition Page 11 of 21
  • 12. Huddersfield Astronomical Society Helium Sodium Oxygen Other 42% 42% 15% 1% OBSERVING MERCURY Mercury is one of the hardest planets to see. Most of the time it is lost in the glare of the sun and only on a few occasions each year does it become visible. VENUS, OUR SISTER PLANET Venus is the second closest planet to the sun. Viewed from earth it is the brightest of the planets. In fact if you know where to look it is possible to see it in broad daylight. At night it is bright enough to cast shadows. It's brightness is due to two things. Firstly it is quite close to earth compared to the other planets. Secondly it is highly reflective and a lot of sunlight bounces off the planet. Venus was once thought to be a tropical planet rich in vegetation. It was difficult to find out what the surface of Venus was like because it was always covered in cloud. In the 1970s American and Russian probes were sent through the clouds to measure surface temperature and analyse the atmosphere. None of these probes lasted very long and the reasons soon became clear. The temperature at ground level was 840 degrees fahrenheit, the atmospheric pressure was 90 times that of earth and the atmosphere was highly corrosive. Basically if you send a probe to Venus it will either melt, be crushed or rust. Not somewhere you would want to park your car! The extremely high temperatures on Venus seem to have been caused by the high quantities of carbon dioxide in the atmosphere which has lead to the greenhouse effect, so an understanding of our sister planet could have profound importance for the future of our own. Page 12 of 21
  • 13. Huddersfield Astronomical Society Venus Statistics Venus Equatorial radius (km) 6,051.8 Equatorial radius (Earth = 1) .94886 Mean distance from the Sun (km) 108,200,000 Mean distance from the Sun (Earth = 1) 0.7233 Rotational period (days) -243.0187 Orbital period (days) 224.701 Mean surface temperature 482°C Atmospheric pressure (bars) 92 Atmospheric composition Carbon dioxide Nitrogen Trace amounts of: Sulfur dioxide, water vapor, carbon monoxide, argon, helium, neon, hydrogen chloride, and hydrogen fluoride. 96% 3+% OBSERVING VENUS With the unaided eye or with binoculars Venus appears like a very bright star. With a small telescope you will be able to see Venus go through phases (similar to our moon) as its position in relation to the sun changes. MARS, THE GOD OF WAR Mars is the last of the rocky inner planets. When seen in the night's sky Mars has a reddish colour and to the Roman's this fiery planet was the bringer of war. Mars has probably caught the human imagination more than any other planet. In the 1870s it was incorrectly reported that canals had been observed on the Martian surface. Since then there has been much speculation about life on Mars. Page 13 of 21
  • 14. Huddersfield Astronomical Society Although the canals never existed, Mars does have some interesting natural surface features. The reddish appearance of Mars is caused by red dust scattered across the Martian surface. Winds sometimes whip up this dust and the entire planet can sometimes be enveloped in a red dust storm. Ironically, it was during one of these storms, that astronomers found some of the most prominent features of Mars. When the Mariner 9 space probe orbited Mars in 1971, the planet was in the midst of a large storm. As the storm subsided, several holes appeared in the dust clouds. Scientists realized that they were looking at several huge volcanoes. The largest of these volcanoes is Olympus Mons (see below). At 15 miles high it is nearly three times the height of Everest and comparable in area to the size of Poland. Page 14 of 21
  • 15. Huddersfield Astronomical Society Mars Statistics Mars Equatorial radius (km) 3,397.2 Equatorial radius (Earth = 1) 5.3264e-01 Mean distance from the Sun (km) 227,940,000 Mean distance from the Sun (Earth = 1) 1.5237 Rotational period (hours) 24.6229 Rotational period (days) 1.025957 Orbital period (days) 686.98 Minimum surface temperature -140°C Mean surface temperature -63°C Maximum surface temperature 20°C Atmospheric pressure (bars) 0.007 Atmospheric composition Carbon Dioxide (C02) Nitrogen (N2) Argon (Ar) Oxygen (O2) Carbon Monoxide (CO) Water (H2O) Neon (Ne) Krypton (Kr) Xenon (Xe) Ozone (O3) 95.32% 2.7% 1.6% 0.13% 0.07% 0.03% 0.00025% 0.00003% 0.000008% 0.000003% OBSERVING MARS With the naked eye Mars will appear as a bright star with a reddish hue. Viewed through a telescope Mars will appear as a bright orange but small disk. It should be possible in a small telescope to see one or both of the Martian polar caps. As Mars reached opposition in 2001 this was the closest we have been to the planet for over 10 years. Page 15 of 21
  • 16. Huddersfield Astronomical Society JUPITER, LORD OF THE PLANETS Jupiter is the giant of the solar system. Its mass is over two times that of all the other planets, moons, asteroids and comets put together. All the inner planets (Mercury, Venus, Earth and Mars) are largely made of rock. Jupiter is the first of the gas giants. It mainly consists of hydrogen and helium. Unlike the inner planets Jupiter doesn't have a surface as such, just layers of cloud held together by the planet's gravity. When viewed through a small telescope it is possible to see dark stripes running parrellel to Jupiter's equator. These are caused by different belts of cloud in the Jovian atmoshere. In a larger telescope, the Great Red Spot can be seen. This was first spotted by Giovanni Cassini in 1665 and appears to be a vast storm that has been raging on Jupiter for hundreds of years. When looking at Jupiter through binoculars, it should be possible to see one or more of the Galilean moons. Discovered by Galileo in 1610 they are the four largest of Jupiter's satellites. The smallest is Europa, which is slightly smaller than our own moon. Io is bigger than our moon and Ganymede and Callisto are both bigger than the planet Mercury. Astronomers are particularly interested in Europa because underneath it's icy surface there appears to be evidence of a warmer salty ocean. If this is the case then Europa could be our best chance of finding extra terrestrial life. Jupiter Statistics Jupiter Equatorial radius (km) 71,492 Mean distance from the Sun (km) 778,330,000 Mean distance from the Sun (Earth = 1) 5.2028 Rotational period (days) 0.41354 Orbital period (days) 4332.71 Mean cloud temperature -121°C Atmospheric pressure (bars) 0.7 Atmospheric composition Hydrogen 90% Page 16 of 21
  • 17. Huddersfield Astronomical Society Helium 10% OBSERVING JUPITER With the unaided eye Jupiter appears as a very bright star. With binoculars it should be possible to see one or more of Jupiter's largest four moons strung out along it's axis. Viewed through a telescope you should be able to see the equatorial belts quite clearly. All the Galilean moons can be seen (providing they are not hidden by the planet). If one of the moons is moving across the front of the planet (called a transit), this may also be visible in a small telescope. The times of transits and the positions of the moons can be found in various astronomy magazines. SATURN THE RINGED PLANET Way out beyond Jupiter is the second of the gas giants, Saturn. When Galileo focused his telescope on Saturn he was puzzled to find that it had an irregular shape. Unfortunately the quality of his telescope meant that Galileo could only guess at what caused this effect. As the quality of telescopes improved later in the century, astronomers were able to solve the puzzle and for the first time see the rings of Saturn. Saturn is situated in the icy depths of the solar system and the rings consist of ice and ice covered rocks. It is possible that the rings are the remnants of an icy moon that was destroyed by a collision with a comet or they could be the leftovers of the cloud of ice and gas that originally formed Saturn. Saturn has more moons than any other planet in the solar system. It's largest moon, Titan, is also the only moon in the solar system that has an atmosphere. It is permanently surrounded by orange cloud that releases Methane rain onto it's surface. Saturn Statistics Saturn Equatorial radius (km) Page 17 of 21 60,268
  • 18. Huddersfield Astronomical Society Mean distance from the Sun (km) 1,429,400,000 Mean distance from the Sun (Earth = 1) 9.5388 Rotational period (hours) 10.233 Orbital period (years) 29.458 Mean cloud temperature -125°C Atmospheric pressure (bars) 1.4 Atmospheric composition Hydrogen Helium 97% 3% OBSERVING SATURN With the naked eye Saturn appears as a stellar like object. Viewed through a telescope Saturn is one of the most beautiful objects in the night sky. Even in a small telescope the rings should be easily visible. On a good night you should also be able to see the shadow of the planet across the rings behind it. In the same plane as the rings a star will be visible to the side of the planet. This is Titan, the largest of Saturn's many moons. THE OUTER PLANETS THE DISCOVERY In 1781 William Herschel while viewing the sky from his garden in Bath, England recognized that an object in the constellation of Gemini was moving against the background of stars. At first he thought he was looking at a new comet but upon further investigation realized that he was looking at a new planet. Herschel named his discovery 'the Georgian planet' after his patron George III. Other names proposed included Herschel, Uranus or Hypercronius. Eventually Uranus became the universally accepted name. By the early 19th century it became evident that the orbit of Uranus did not follow Newton's law of Gravitation. Many astronomers began to question whether Newton's theory of gravitation applied to an object so far from the sun. However two astronomers, John Couch Adams in England and Urbain Le Verrier in France, both independently came Page 18 of 21
  • 19. Huddersfield Astronomical Society up with the theory that the orbit of Uranus had been disturbed by a more distant planet. Working to Le Verrier's calculations, astronomers at the Berlin observatory were able to identify this planet. They had discovered the eighth planet of the solar system, Neptune. After the discovery of Neptune, astronomers started to look further into the depths of the solar system for a ninth planet. In 1930, an American astronomer discovered the last of the known worlds of our solar system, Pluto. Pluto is named after the god of the underworld, an apt name for a planet that orbits 5900 million kilometres from the sun. THE THREE NEW WORLDS Uranus is a bluish green gas giant. It takes 84 years to orbit the sun. Unlike all the other planets Uranus spins on its side. This means that the northern hemisphere of the planet sees the sun continually spinning round in the sky for over 40 years only to be plunged into darkness for 40 more. For the southern hemisphere the seasons are reversed. Uranus Statistics Pluto Equatorial radius (km) 25,559 Mean distance from the Sun (km) 2,870,990,000 Mean distance from the Sun (Earth = 1) 19.1914 Rotational period (hours) -17.9 Orbital period (years) 84.01 Mean cloud temperature -193°C Atmospheric pressure (bars) 1.2 Atmospheric composition Hydrogen 83% Helium 15% Methane 2% Neptune is a blue (gas giant) planet of similar size to Uranus. Its colour comes from the methane in its atmosphere. Methane absorbs the longer visible wavelengths (red and yellow) so the reflected sunlight appears blue. Page 19 of 21
  • 20. Huddersfield Astronomical Society Neptune has a very active atmosphere with high wind speeds that move around the planet faster than it rotates. Neptune Statistics Neptune Equatorial radius (km) 24,746 Mean distance from the Sun (km) 4,504,300,000 Mean distance from the Sun (Earth = 1) 30.0611 Rotational period (hours) 16.11 Orbital period (years) 164.79 Mean cloud temperature -193 to -153°C Atmospheric pressure (bars) 1-3 Atmospheric composition Hydrogen 85% Helium 13% Methane 2% Pluto is a bit of a puzzle. All the inner planets are small rocky worlds. The outer planets are gas giants. However Pluto, the furthest of the planets from the sun is a small icy rock of a planet. It is almost a double planet system because it is orbited by a large moon, Charon. The discovery of the moon Charon weakened the theory that Pluto is an escaped moon of Neptune. It is now thought that Pluto came from a region of space debris left after the forming of the solar system called the Kuiper Belt. Pluto Statistics Pluto Equatorial radius (km) 1,160 Mean distance from the Sun (km) 5,913,520,000 Mean distance from the Sun (Earth = 1)) 39.5294 Rotational period (days) -6.3872 Orbital period (years) 248.54 Atmospheric composition Methane 0.3 Nitrogen OBSERVING THE PLANETS Arm yourself with a finder chart (from a computer star chart or astronomy magazine) and a pair of binoculars and hopefully you will be able to find Uranus edging it's way across the background of stars. For beginners this is quite a difficult task because the planet is usually not bright enough to be seen with the unaided eye. Neptune is an even harder target to find than Uranus. Although it is roughly the same size as Uranus, Neptune is a lot further away and therefore appears a lot dimmer and smaller to us. Pluto is only visible in the largest of telescopes and is beyond the reach of most amateur astronomers. Page 20 of 21
  • 21. Huddersfield Astronomical Society Page 21 of 21

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